The Oscar-winning actress, 34, has two children -- Ava and Deacon -- with her first husband, actor Ryan Phillipe. She also enjoyed a high-profile, two-year relationship with actor Jake Gyllenhaal, who is currently dating Taylor Swift.
The announcement is the third celebrity holiday engagement, after Natalie Portman (who is also pregnant) to Benjamin Millepied and LeAnn Rimes to Eddie Cibrian.
Toth is an agent with the Hollywood powerhouse CAA. No wedding date has been announced.
"The couple is extremely happy," her rep said in a brief statement.
Reese recently spoke about aging, and her sexuality, in a cover interview for Glamour magazine.